[Driver] Pass +reserve-x25 to backend if -ffixed-x25 is specified

This was accidentally omitted in r353957 breaking the Clang test.

llvm-svn: 353960
diff --git a/clang/lib/Driver/ToolChains/Arch/AArch64.cpp b/clang/lib/Driver/ToolChains/Arch/AArch64.cpp
index dc4a201..e229e4c 100644
--- a/clang/lib/Driver/ToolChains/Arch/AArch64.cpp
+++ b/clang/lib/Driver/ToolChains/Arch/AArch64.cpp
@@ -373,6 +373,9 @@
   if (Args.hasArg(options::OPT_ffixed_x24))
     Features.push_back("+reserve-x24");
 
+  if (Args.hasArg(options::OPT_ffixed_x25))
+    Features.push_back("+reserve-x25");
+
   if (Args.hasArg(options::OPT_ffixed_x26))
     Features.push_back("+reserve-x26");